The human health costs of the obesity epidemic have been well documented, but a new study is shedding light on another set of victims: horses mounted by overweight riders.

One-third of recreational riders are too obese for their steeds, putting the animals at risk for health problems including lameness and back pain, reports the U.K.'s Daily Mail, which cited a study from the Journal of Veterinary Behavior.

That may lead to higher health care costs for horses, which can already be considerable. While amounts vary, horse owners typically spend about $300 a year on routine health care for their animals, although that can increase to thousands if a horse is injured or ill, according to Equine.com.

Overweight riders can also cause the animals to develop behavioral problems, such as bucking and rearing, the Mail notes. (Whether the horses are trying to get the overweight riders off their backs or are just annoyed, the article doesn't say.)

"People tend to think horses are such big animals they must be okay, and not to take notice of the weight issue of riders. But the health impact on the horse can be quite extreme, quite quickly," Hayley Randle, one of the study authors, told the newspaper.

Recreational riding adds $11.8 billion in spending to the U.S. economy, with 2 million horse owners in America alone, according to the Equestrian Channel. To be sure, the academic study looked at the U.K., but weighty riders are likely an issue for American horses as well, given that the U.S. is the third-fattest country on earth, while the U.K. comes in at No. 10.

Adding to the problems of overburdened horses, plus-size saddles are hitting the market. The WOW "Bounty" saddle has a seat that's "designed to offer supreme support and comfort to the larger rider,” according to Horse and Hound. Another company is also working on a saddle for heftier behinds.

A rider shouldn't weigh more than 10% of her horse's weight, the Mail notes, citing vet guidelines. But weight ratios aren't commonly known in the equine community, Randle said. She added, "People do seem generally to be a bit heavy for horses. That is just a consequence, I suppose, of our average weights going up."

Heavy riders sometimes pepper message boards with questions about which horses are best to carry weight (draft horses are frequently mentioned as potential steeds). When one woman asked what horse her 350-pound husband should ride, one forum respondent wrote, "Develop an interest in driving."